The preparation of polyether alcohols is described in M. Ionescu, “Chemistry and technology of polyols for polyurethanes”, Rapra Technology, 2005.
Frequently used alkylene oxide starting materials for the preparation of polyether alcohols are propylene oxide (PO) and/or ethylene oxide (EO).
In the preparation of PEOLs, in particular in the batchwise preparation, variations from batch to batch occur every now and again. The variations are noticeable in flexible foam polyols, especially in the concentration of the unsaturated monools and the molecular weight distribution, and have an effect on the processing and the mechanical properties of the polyurethanes produced therefrom. In the case of rigid foam polyols, excessively high sugar concentrations are often observed and cause deposits on the foaming machines in the processing of polyurethanes. If, for the purposes of quality control, Shewhart control cards are used to check the production process, infringements of the six sigma rule and the trend rules, for example the Western Electric rule, occur. The procedure is described, for example, in Douglas C. Montgomery, Introduction to Statistical Quality Control, 6th edition 2008, Wiley & Sons, New York, ISBN 0470169923. Deviations from these rules indicate that systematic, not random, fluctuations are present in the process, which means that the production process is incompletely controlled.
These problems can in many cases be attributed to insufficient mixing of the contents of the reactor.
Insufficient mixing also results in the following disadvantages:    a) the alkylene oxide which is present in gaseous form under the customary reaction conditions does not come into contact uniformly with all reactants, i.e. essentially the starters, which results in discrimination in the buildup of the chain and thus fluctuating product properties,    b) gas/liquid mass transfer is made more difficult, which causes increased reaction and reactor occupation times,    c) heat transfer is hindered; if this is not taken into account in the mode of operation, undesirable overheating of the reaction mixture, in particular local overheating, occurs, or when it is taken into account, the rate of introduction of alkylene oxide has to be reduced, which increases the reaction time and thus the reactor occupation time,    d) in the case of overheating due to c), when a DMC catalyst is used it can be partially or completely deactivated; particularly in the case of KOH catalysis, the content of unsaturated constituents, viz. monools, can increase;    e) in the case of sugar polyols, the residual sugar content in the finished polyol can be increased and cause turbidity of the product,    f) in addition, overheating can lead to an unstable operating state, which in the extreme case can result in a runaway reaction.
The problems mentioned have hitherto not been solved satisfactorily in the available literature, e.g. in M. Ionescu, Chemistry and Technology of Polyols for Polyurethanes, Rapra Technology Limited, Shawbury/UK, 2005, p. 336 ff., and in M. Baerns, A. Behr, A. Brehm, J. Gmehling, H. Hofmann, U. Onken, Ullmann's Encyclopedia of Industrial Chemistry, New York, 5th ed., Vol. A21, p. 665.
It is therefore an object of the invention to provide a process for preparing polyether polyols which avoids the abovementioned disadvantages as far as possible.